Add tutorial of free flow over porous media

free-flow-over-porous-media/README.md

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+---
+title: Free flow over porous media 2D
+permalink: tutorials-free-flow-over-porous-media-2d.html
+keywords: DuMux, porous media
+summary: Flow-flow coupling example with porous media field and free flow field.
+---
+
+{% note %}
+Get the [case files of this tutorial](https://github.com/precice/tutorials/tree/master/free-flow-over-porous-media-2d). Read how in the [tutorials introduction](https://precice.org/tutorials.html).
+{% endnote %}
+
+## Setup
+
+This tutorial solves a simple coupled system consisting of a one-phase free flow and a one-phase flow in a porous media.
+
+A pressure gradient is applied to the free flow domain from left to right, while at the top of the free-flow we have a non-permeable wall with no-slip boundary conditions. In the porous media, we assume no-flow across the domain boundaries (left, bottom and right boundary). At the interface we assume a no-slip condition.
+
+The setting is illustrated in the following figure:
+
+![Free flow over porous media setup](images/tutorials-free-flow-over-porous-media-setup.png)
+
+## Configuration
+
+preCICE configuration (image generated using the [precice-config-visualizer](https://precice.org/tooling-config-visualization.html)):
+
+![preCICE configuration visualization](images/tutorials-free-flow-over-porous-media-precice-config-visualization.png)
+
+## Available solvers
+
+Both the flow in free flow and porous media can be solved using the simulation framework [DuMu<sup>x</sup>](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/).
+
+## Solver setup
+
+To solve the flows with the DuMux framework, the necessary DUNE modules need to be downloaded and set up. This is done by running `sh setup-dumux.sh` in the tutorial folder.
+
+Note that if an existing installation of DUNE modules is detected in a default location, this may lead to problems in running the `setup-dumux.sh` script. The environment variable DUNE_CONTROL_PATH is suppressed by the script.
+
+To recompile only the simulation, run `sh compile-dumux-cases.sh` in the tutorial folder.
+
+## Running the simulation
+
+You can find the corresponding `run.sh`script for running the case in the folders corresponding to the solvers you want to use.
+
+To run the free-flow participant, run:
+
+```bash
+cd free-flow-dumux
+./run.sh
+```
+
+To run the porous-media participant, run:
+
+```bash
+cd porous-media-dumux
+./run.sh
+```
+
+So far only serial execution is supported for this tutorial, which takes less than two minutes to finish.
+
+## Post-processing
+
+The VTU files from both solvers could be rendered and inspected with ParaView.
+
+## Further information
+
+The simulation is stationary and the results of the pressure and the velocity fields are as follows:
+
+![Free flow over porous media results - pressure](images/tutorials-free-flow-over-porous-media-result-pressure.png)
+![Free flow over porous media results - velocity](images/tutorials-free-flow-over-porous-media-result-ux.png)
+
+Each solver folder contains an input file (`params.input`) that will be passed to the solver executables. This is a DuMUX input file describing the simulation setting, e.g., pressure, mesh size, time stepping, etc.

free-flow-over-porous-media/clean-tutorial.sh

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+../tools/clean-tutorial-base.sh

free-flow-over-porous-media/compile-dumux-cases.sh

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+#!/usr/bin/env sh
+set -e -u
+
+cd free-flow-dumux/build-cmake/appl
+make free_flow_dumux
+cd ../../../porous-media-dumux/build-cmake/appl
+make porous_media_dumux
+cd ../../../
+
+# Move free-flow-dumux and porous-media-dumux executables to the participant folder level
+mv free-flow-dumux/build-cmake/appl/free_flow_dumux free-flow-dumux/
+mv porous-media-dumux/build-cmake/appl/porous_media_dumux porous-media-dumux/

free-flow-over-porous-media/free-flow-dumux/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.13)
+project(free-flow-dumux CXX)
+
+if(NOT (dune-common_DIR OR dune-common_ROOT OR
+      "${CMAKE_PREFIX_PATH}" MATCHES ".*dune-common.*"))
+    string(REPLACE  ${PROJECT_NAME} dune-common dune-common_DIR
+      ${PROJECT_BINARY_DIR})
+endif()
+
+#find dune-common and set the module path
+find_package(dune-common REQUIRED)
+list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/modules"
+  ${dune-common_MODULE_PATH})
+
+#include the dune macros
+include(DuneMacros)
+
+# start a dune project with information from dune.module
+dune_project()
+
+dune_enable_all_packages()
+
+add_subdirectory(appl)
+
+# finalize the dune project, e.g. generating config.h etc.
+finalize_dune_project(GENERATE_CONFIG_H_CMAKE)

free-flow-over-porous-media/free-flow-dumux/appl/CMakeLists.txt

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+add_executable(porous_media_dumux main.cc)
+target_compile_definitions(porous_media_dumux PUBLIC "ENABLEMONOLITHIC=0")
+target_link_libraries(porous_media_dumux PRIVATE dumux-precice)
+
+add_input_file_links()